I have spoken with the Phone carrier Technical Support team and they have given we the following response.
This is definitely a server issue.

Basically when an email is received it is given a Unique Identifier by the system. When two devices are trying to access the same UID at the same time the server has to decide which one gets it. In this case, its not our customer. So they need to amend the conflict resolution rules on the server.

Exchange administrators are always vigilant about Exchange crashes and disasters that are possible any time. It is quite essential to identify the symptoms of a possible Exchange issue and be prepared with a proper recovery plan. There are multiple…

Exchange organizations may use the Journaling Agent of the Transport Service to archive messages going through Exchange. However, if the Transport Service is integrated with some email content management application (such as an antispam), the admini…